The investigation outlined in this proposal is designed to improve our knowledge of critical parameters of radiation mutagenesis in human cells. The issues that we have selected for investigation, low dose effects, the influence of cellular repair proficiency, and potential differences among various genetic loci, have each been recognized for several decades but their role in radiation mutagenesis remains uncertain. Each of these critical parameters may substantially influence the observed mutational spectrum in a manner which cannot be accurately predicted at this time. Mutational spectra may be defined as the complete set of molecular events which can result in the expression of a mutant phenotype. This includes the distribution and PM of DNA sequence alterations which occur within a target locus. In addition, loss of heterozygosity (LOH) at previously heterozygous loci may occur by one of several possible mechanisms and account for a large fraction of all mutations occurring at such loci. These events are also essential elements of a mutational spectrum since they are responsible for a large proportion of observed mutant phenotypes. This proposal is designed to use mutational spectra, as broadly defined here, to coordinately investigate the critical parameters outlined above. Our specific objectives include: 1) the definition of mutational spectra from very low dose x-ray exposure (10 cGy) in TK6 human lymphoblasts; 2) determination of mutational spectra in cell lines derived from radiosensitive Ataxia Telangiectasia patients and their heterozygous relatives; 3) determination of comparative mutational spectra at three endogenous loci available for study in TK6 human lymphoblasts. Mutants will be collected by parallel selections from the same irradiated populations. The objective of the research proposed in this application is to develop a better understanding of the cellular, molecular and dosage factors which determine the spectrum of mutations induced by x-rays in human B lymphoblastoid cell lines. Improved understanding of the influence of these factors in radiation mutagenesis of human cells will permit a better estimate of the risk to human populations from low dose radiation exposure.